Non trait object safe return diagnostic needs improvement

[hythloday]

Here's some illegal code that generates a very reasonable error message:

trait Foo {
  fn apply() -> i64;
}

struct Baz{}
static BAZ: Baz = Baz{};
impl Foo for Baz {
    fn apply(&self) -> i64 { 4 }
}

// <anon>:8:5: 8:33 error: method `apply` has a `&self` declaration in the impl, but not in the trait [E0185]

Adding a function that references this broken trait changes the error message and is imho quite misleading:

fn lookup() -> Box<&'static Foo> {
    Box::new(&BAZ)
}

<anon>:11:1: 13:2 error: the trait `Foo` cannot be made into an object [E0038]
<anon>:11 fn lookup() -> Box<&'static Foo> {
<anon>:12     Box::new(&BAZ)
<anon>:13 }
<anon>:11:1: 13:2 help: see the detailed explanation for E0038
<anon>:11:1: 13:2 note: method `apply` has no receiver

The error code has changed from something that pinpoints exactly where I went wrong to something unrelated with advice that won't help me see my problem, and highlights the wrong place (the function that's returning this trait rather than the trait itself). Ideally the error message wouldn't change at all.

rustc 1.10.0-nightly (8da2bcac5 2016-04-28)
binary: rustc
commit-hash: 8da2bcac5db1e091b90cceb19d0496f0f7501c88
commit-date: 2016-04-28
host: x86_64-apple-darwin
release: 1.10.0-nightly

(happens on stable and beta too)

[arielb1]

The error message is correct - the trait Foo is not object-safe, as the method apply has no receiver. The impl has nothing to do with it.

Maybe we should use "self-type" instead of "receiver"?

[arielb1]

Again: your declared a perfectly fine trait with a single static method. Then you wrote both a non-matching impl and a non-matching method. rustc reported the method, but not the impl.

[hythloday]

No dispute that the error message is correct (or at least, not incorrect). My feeling is that it is unhelpful - the code, whether in the presence or absence of lookup, is erroneous in the same way, and the error message in the first example is the correct one to emit even in the presence of lookup. No changes to lookup can be made that will make this code compile, so it's unhelpful to highlight it as the erroneous section.

It is strange to see one error message refer to a receiver and the other refer to a '&self' declaration but that's not essential.

Hope that's clearer as to why I regard this as a bug. Thanks for the reply!

[Aatch]

@hythloday they're different kinds of errors. The first one is reporting the invalid method in the impl, the second error is actually a trait-object error. Moreover, the error is pointing to where the error is occurring, as that should ideally provide more context. The compiler doesn't know that you actually meant to write fn apply(&self) in the trait or if you actually meant to use a different trait instead. If the trait isn't local, we won't even have a span to highlight anyway.

Anyway, these are two different errors, both are equally correct. They're just reporting different things and one of them isn't being shown.

[estebank]

The current output points out at the problem:

error[E0185]: method `apply` has a `&self` declaration in the impl, but not in the trait
 --> <anon>:8:5
  |
2 |   fn apply() -> i64;
  |   ------------------ trait declared without `&self`
...
8 |     fn apply(&self) -> i64 { 4 }
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `&self` used in impl

I believe this can be closed. CC @jonathandturner

[sophiajt]

@estebank - the original report I think is talking about the bad error given for:

trait Foo {
  fn apply() -> i64;
}

struct Baz{}
static BAZ: Baz = Baz{};
impl Foo for Baz {
    fn apply(&self) -> i64 { 4 }
}

fn lookup() -> Box<&'static Foo> {
    Box::new(&BAZ)
}

We still give the same confusing error:

error[E0038]: the trait `Foo` cannot be made into an object
  --> quickie.rs:11:1
   |
11 |   fn lookup() -> Box<&'static Foo> {
   |  _^ starting here...
12 | |     Box::new(&BAZ)
13 | | }
   | |_^ ...ending here: the trait `Foo` cannot be made into an object
   |
   = note: method `apply` has no receiver

[estebank]

Current output:

error[E0038]: the trait `Foo` cannot be made into an object
  --> src/lib.rs:11:1
   |
11 | fn lookup() -> Box<&'static Foo> {
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `Foo` cannot be made into an object
   |
   = note: method `apply` has no receiver

[estebank]

Current output

error[E0038]: the trait `Foo` cannot be made into an object
  --> src/lib.rs:11:1
   |
2  |   fn apply() -> i64;
   |      ----- associated function `apply` has no `self` parameter
...
11 | fn lookup() -> Box<&'static Foo> {
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `Foo` cannot be made into an object

[estebank]

After #68347 the output would be:

error[E0038]: the trait `Foo` cannot be made into an object
  --> file.rs:11:16
   |
1  | trait Foo {
   |       --- this trait cannot be made into an object...
2  |   fn apply() -> i64;
   |      ----- ...because associated function `apply` has no `self` parameter
...
11 | fn lookup() -> Box<&'static Foo> {
   |                ^^^^^^^^^^^^^^^^^ the trait `Foo` cannot be made into an object
   |
help: consider turning `apply` into a method by giving it a `&self` argument or constraining it so it does not apply to trait objects
   |
2  |   fn apply() -> i64 where Self: Sized;
   |                     ^^^^^^^^^^^^^^^^^

[hythloday]

Thanks Esteban! That is a lot clearer and exactly what I would expect.